Such a radiation detection device in the form of a pyranometer is known from U.S. Pat. No. 3,876,880. In this publication a pyranometer is described in which an external pump and heat exchanger provide a cooling air stream to the detector housing for cooling the inner and outer domes. In addition, an external blower directs an air stream across the domes for increasing the radiation measurement accuracy.
It is commonly known in pyranometers (for instance model SR11, marketed by Hukseflux Thermal Sensors B.V., Delft, Netherlands) to utilize an inner and outer glass dome overlying the sensor. The two glass domes reduce wind related signal noise and thermal offset related error effects, resulting in improved measurement accuracy.
Zero offsets and deposition of water on the instrument dome are important factors determining the reliability of measurements with pyranometers.
Pyranometers that comply with the ISO 9060 standard must have a flat spectral response in the 0.3 to 3 micron spectral range. They employ thermo-electric sensors. Using thermal-electric sensors (as opposed to photo-electric sensors), zero offsets, i.e. signals not related to the quantity to be measured, are a significant source of measurement uncertainty. Reduction of zero offsets is useful because this improves measurement accuracy.
The most significant offset is the sensitivity to far-infra-red radiation exchange, the “zero offset A”, as defined by the ISO 9060 standard which classifies pyranometers. Zero offset A is caused by the outer dome cooling down by radiation exchange with the sky, which is a relatively cold source of far-infra-red radiation. The balance of the far-infra-red radiation exchange from the outer dome to the sky is negative. The WMO manual and ISO 9060 define a reference condition of −200 W/m2, representing worst case conditions. The outer dome turns cold, and on its turn cools down the inner dome by the same mechanism of radiation exchange. Finally the sensor produces a negative offset by its radiation exchange with the inner dome.
There are other significant offsets such as “zero offset B”, defined as the offset caused by heating or cooling the instrument with a fixed temperature rate of change of 5 K/hr. This temperature change produces internal temperature differences in the instrument. These differences not only cause far-infra-red radiation exchange but also generate energy flows to or from the sensor. Both mechanisms generate zero offsets adding up to zero offset B.
Heating a pyranometer, for example by using an electrical resistor or by externally applied heated ventilation air may independently produce zero offsets by the same mechanisms that cause zero offset B. Offsets caused by heating are not specifically mentioned or defined in the ISO 9060 standard. In practice they are an integral part of the measurement, and therefore part of the measured zero offset A and zero offset B. For one instrument model there may be offset A and B specifications with heating and without heating.
Deposited water on pyranometer domes leads to unpredictable and potentially very large but non-quantifiable errors; it reduces the “data availability”. Deposition of rain and snow are quite common, but this usually goes together with cloudy conditions under which the measurement errors are small. Most pyranometers are located in moderate climate zones. Deposition of dew or frost on dome in the early morning regularly causes large errors. Water condenses on the dome because at night by far-infra-red radiation exchange with the sky these cool down to a temperature below dew point.
A pyranometer with water deposited on the dome operates beyond its rated conditions. Prevention of deposition of water or fast removal of deposited water is useful because a dry dome is the rated condition for a reliable measurement. A dry dome also is unattractive for dust to stick to.
Lower zero offsets of thermo electric sensors may be attained by improving thermal coupling between the thermal sensor, the instrument metal body and the inner window. Better thermal coupling results in smaller temperature differences between these parts and thereby to reduced far-infra-red radiation exchange.
For example, the model CMP22 pyranometer attains lower zero offsets than the otherwise equivalent model CMP11 by using two quartz domes with a higher thermal conductivity and larger thickness than the CMP11 domes. Zero offsets are reduced by a factor 2.
A second example is that the inner dome acts as a radiation shield between the outer dome and the sensor, blocking radiation far-infra-red radiation exchange. By adding the inner dome to a pyranometer only employing a single outer dome, the far-infra-red radiation exchange from the dome to the sensor, and thus zero offset A, is reduced by a factor of approximately 1.5. This is illustrated by comparing zero offset A specifications of pyranometer models CMP11 and CMP3.
As a third example, high wind speed or artificial ventilation may reduce zero offset A by promoting thermal coupling between the pyranometer body and outer dome. For zero offset B, the part of the zero offset caused by energy flows to or from the sensor may be reduced by symmetrically coupling a sensor to the instrument body, or by using a sensor with a low heat capacity. Some sensors employ a so-called compensation element.
Heating a pyranometer dome may help prevent dew and frost. A heated dome should have a temperature above dew point, so that moisture in the ambient air does not condense on it. In case water is deposited, heating accelerates evaporation of dew and rain, and promotes the process of sublimating or melting of snow and frost. To promote sublimation and melting, higher temperatures are beneficial. Melting requires a dome temperature above 0° C.
The simplest option would be to directly heat a pyranometer, i.e. internally or with a heater connected to the instrument body, as opposed to externally via ventilation air. Using traditional pyranometers, already at low power levels, where heating is not yet effective to prevent humidity from condensing on the instrument dome, the added zero offsets caused by direct heating are beyond the specification limits of the ISO 9060 standard. The standards covering pyranometer use such as ISO TR 9901 therefore do not mention direct heating as a possibility. In some cases direct heating is nevertheless used, for instance in pyranometer model SR20, where it is typically switched on at night only when offsets do not matter because there is no sun. The zero offset caused by 1.5 W direct heating is −8 W/m2 which is beyond the specification limits of ISO 9060 for the accuracy class. Applying direct heating at higher power, for example to promote evaporation and sublimation or to melt snow or ice is possible, but creates still larger errors and therefore is not mentioned in any standard.
The present invention aims at improving the known detector construction by further reducing the zero offset while providing improved performance under moist and/or icy conditions at a low use of power.
Low power consumption is essential for use of pyranometers as these instruments are often applied in remote locations where mains power is not available.
The invention in particular aims at improving the thermal coupling between the sensor, the instrument metal body and the domes.
The invention furthermore aims at providing a detector with low maintenance and low power requirements and large data availability.